1. Field of the Invention
The present invention pertains to apparatus and methods for handling fluids, and more particularly to apparatus and methods for charging a compressed fluid into or discharging the compressed fluid out of containment, including into and out of a storage or transport container.
2. Description of the Related Art
Gases are stored and transported under pressure to reduce the volume of the gas. Numerous gases, including air, nitrogen, oxygen, hydrogen, ethane and propane, are compressed for storage and transport, and natural gas is a gas commonly compressed. The term “natural gas” as used in this document refers to light hydrocarbon compositions that are dominated by the methane molecule, but may be comprised of heavier hydrocarbon molecules as well as limited non-hydrocarbon impurities (such as water, carbon dioxide, and nitrogen) in any proportion that would exist as gas vapor at ambient temperature and pressure. This “natural gas” may have originated as a naturally occurring fluid stream extracted from the earth or as synthetically combined mixture of molecules created for the purposes of storage and/or transport in or on some form of mobile platform (such as a ship, railcar, or truck trailer). Compressed natural gas may be referred to simply as CNG, whether refrigerated or not. Pressurized liquid natural gas or pressurized natural gas liquid, often referred to as PLNG herein, is deeply refrigerated but may not necessarily be stored at temperatures below the critical temperature of methane.
Compressed fluids have been typically contained in relatively (or very) long cylinders, bullet tanks, oblate spheroids or spheres that all feature at least one circular cross-section cut and have relatively high wall thickness to diameter ratios (t/D) to resist the pressure-induced hoop stresses because the materials used to fabricate these essentially rigid containers are of limited effective strength in the orientation of the critical stresses (whether metallic or fiber-based matrix shells or hybrid composites thereof).
Many advancements have been made that concern storing and transporting compressed fluids efficiently. See, for example, the following patents and patent applications, each of which is incorporated by reference: U.S. Pat. No. 6,584,781, issued to Bishop et al., U.S. Pat No. 6,655,155, issued to Bishop, and U.S. Pat. No. 6,725,671, issued to Bishop, and U.S. Patent Application Pub. Nos. 20020046547, filed by Bishop et al. and assigned Ser. No. 09/943,693 and 20030106324, filed by Bishop et al. and assigned Ser. No. 10/316,475, each of which is believed to be assigned to Enersea Transport, LLC of Houston, Tex., and are collectively referred to as “the Bishop patents.” The Bishop patents consider that the mass ratio for circular cylinders can be commercially optimized by selecting storage pressures that minimize the compressibility factor Z for temperatures below about −10° C. while keeping the cargo stored as a dense phase fluid. U.S. Pat. Nos. 3,232,725 and 3,298,805 are incorporated by reference, each of which issued to Secord et al. and are collectively referred to as “the Secord patents.” The Secord patents describe temperature and pressure ranges for efficiently containing a compressed natural gas in a dense phase and/or in mixed phase.
Existing art does not adequately address the challenges of efficiently loading and discharging compressed fluids, particularly pressurized, refrigerated natural gas cargos, which would be useful for establishing commercially attractive storage and transport services. If a displacement practice is not adopted for loading and unloading of pressurized gas cargos, expansion and recompression of gases within the containment can result in heat energy build up within storage containers, which impacts the efficiency undesirably. See U.S. Pat. No. 6,655,155, issued to Bishop and incorporated by reference, referred to herein as “Bishop,” which describes a method for using a displacement liquid to load or offload compressed gas from a gas storage system. A cargo displacement system can avoid undesirable aspects of cargo expansion and compression during both loading/charging and unloading/discharging operations.
Such a system requires that the cargo enter and exit primarily through a pathway that serves as both entry and exit while the displacement fluid enters and exits by an opposing pathway. If a liquid is being used to displace a less dense fluid (e.g., CNG or PLNG) cargo, then the cargo enters and leaves primarily by way of top piping connections while the displacement liquid enters and exits by pathways disposed at the bottom of containment. If the displacement fluid is a light gas over a denser cargo fluid, then the cargo will enter and exit by way of a bottom or lower end path (e.g., a nozzle or dip tube). The displacement gas will enter and exit primarily by top end pathways. Under certain conditions (e.g., emergencies), it may be desirable to switch the means of access and egress for the fluids and for either displacement approach.
Bishop uses a cold, non-freezing liquid as the displacement fluid (such as an ethylene glycol and water blend) for loading and discharging cold CNG. Bishop uses a low pressure gas (possibly nitrogen) to push the displacement liquid back out of the tanks after the liquid has displaced the cargo or traps and expands a small gas cap of cargo fluid left on top of the displacement. See U.S. Pat. No. 6,202,707, issued to Woodall et al. and incorporated by reference, which is referred to as “Woodall” herein and U.S. Pat. No. 7,219,682, issued to Agnew et al. and incorporated by reference, which is referred to as “Agnew” herein. In a manner analogous to Bishop, Woodall uses a dense, non-freezing liquid to displace lighter PLNG out the tank tops for discharging PLNG cargo, while Agnew proposes an approach similar to both Bishop and Woodall to displace a warmer compressed natural gas cargo fluid out of topside piping manifolds. Woodall uses submersible pumps inside the tanks to recover and distribute the displacement liquid after it has discharged the cargo from the tanks. Woodall and Agnew both recommend filling the void created in the tanks when the displacement liquid is drained or pumped out and over to a subsequent tank (or tier of tanks) with a low pressure gas, while Woodall further suggests that gas recovered from the discharged cargo stream could be used to fill the void. See also U.S. Pat. No. 6,932,121, issued to Shivers III and incorporated by reference, which is referred to as “Shivers” herein. Shivers proposes injecting a warm gas from a remote salt dome reservoir into the topsides of the tanks to displace a very cold liquid cargo (PLNG) out the bottom side piping connections in a reversal of the processes described by Bishop, Woodall, and Agnew.
The amount of residual gas remaining in storage after normal discharge operations is another issue that challenges existing compressed gas storage and transport concepts because this quantity of gas, frequently called heel, cannot be sold.
If a blow-down method is employed instead of one of the displacement schemes just described, the gas discharge rate will drop as the driving pressure within storage approaches the back pressure of the receiving system and will eventually stop discharging completely when the pressures balance. To overcome this back-pressure problem and avoid leaving commercially punitive quantities of heel gas in the cargo tanks, expensive scavenging compressor elements are inserted into the gas extraction system to allow a draw down from containment to lower pressures. The cost and effectiveness of scavenging compression systems are impacted by the compression ratio needed to push gas into pressurized receiving infrastructures (e.g., a gas transmission pipeline) by raising the pressure of discharging gas from an inlet pressure that is dropping relative to the back pressure imposed by the receiving infrastructure.
The mass of residual or heel gas left in the container is a function of the final pressure, temperature, and composition of the gas within the containers, as determined by the scavenging limits of the extraction system compressor(s), and the temperature of the residual gas. During unloading of refrigerated cargos, the temperature of the gas remaining in storage drops, causing its density to increase. Unless the blow down and scavenging cargo discharge operations take place slowly enough for the residual gas to be re-warmed by heat input from the cargo containers and hold spaces, commercially punitive quantities of cargo gas will remain as heel. In commercial scenarios, it is undesirable to warm a deeply chilled containment system up to ambient temperature and impractical to wait the many days needed to do so.
Bishop, Woodall, and Agnew all proposed a liquid displacement solution that uses a liquid which will not freeze at the targeted operating temperatures when it is being used as a liquid piston to displace the cargo from storage. Liquid displacement makes it possible to remove all cargo gas from storage, but the liquid displacement system incurs high costs for acquiring at least one reservoir of the liquid, refrigeration and insulation to keep it at the proper temperature, and a pumping system to drive the liquid into the containment system against its internal pressure. A recommended displacement liquid like ethylene glycol is good from the perspective that it does not interact with (absorb much of) the cargo gas. However, it is not practical to use for operating temperatures much below −30° C. because of increasing viscosity. Further, costly refrigeration systems must be provided to chill the displacement liquid to overcome heat gained during operations and storage.
Alternatively, Shivers employs the displacement method in reverse by using a warm compressed gas from a salt dome reservoir or existing pipeline infrastructure that has a warm gas supply available to displace the cold liquid cargo from storage. Unfortunately, most locations where natural gas deliveries are needed do not have gas readily available from salt formations or excess pipeline gas supplies available to support the operation described by Shivers and, where such a gas source is available, the gas properties may not be compatible with the cargo gas being delivered. Also, the containment systems and piping onboard the vessels described by Shivers are maintained at very low temperatures to keep the cargo in the liquefied form described by Shivers. Injecting very warm or hot gas directly into those structures will cause significant thermal shock and stresses to critical elements of the Shivers system. Further, Shivers indicates that cargo discharging operations are initiated by injecting this warm gas into the top side connections of the containers. However, large amounts of energy are wasted by initiating delivery by pushing the cargo out at the same pressure under which it has been transported.
Woodall and Shivers both describe storing two-phase fluids according to storage conditions suggested by, for example, Secord in rigid cylindrical containers for transport, but neither provide a means for efficiently discharging the cargo to achieve the most commercially attractive delivery of cargo with minimal residual gas volumes remaining on board the transport vessel. Secord also does not address how to best discharge two-phase fluids from rigid cylindrical containers.
U.S. Pat. No. 6,085,528, issued to Woodall et al., U.S. Pat. No. 6,460,721, issued to Bowen et al., and U.S. Pat. No. 7,147,124, issued to Minta et al. describe high strength cylinders for transporting PLNG at low temperatures, but do not describe an efficient way to extract the cargo and minimize mass of the residual product in storage upon delivery to the receiving facilities. Shivers describes a method for driving the PLNG out with very warm gas, but the method is inefficient and limited in its application to very specific delivery points where the warm gas can be collected from a remote source.
U.S. Pat. No. 6,339,996, issued to Campbell and incorporated by reference, which is referred to as “Campbell” herein, and U.S. Pat. No. 5,803,005, issued to Stenning and Cran and incorporated by reference, which is referred to as “Stenning” herein, both address means for blowing gas into and out of high pressure, ambient temperature CNG cylinders. Neither inventor is addressing the approach or systems required to optimize storage efficiency for natural gas cargos at very low temperatures. Campbell addresses means to balance load distribution within a CNG transport ship by moving liquids around between the bottoms of selected individual tanks or banks of tanks via dedicated liquid manifolds connected to the bottom side of vertical CNG cargo cylinders, but does not address the losses of cargo storage efficiency due to blow-in and blow-down of the described cargo fluids. Stenning makes an effort to limit some of the problems with blow-in and blow-down operations, but only addresses ways to move ambient or near-ambient temperature CNG in and out of the tops of CNG cylinders and does not address issues involving substantial liquid quantities therein.
The greater the mass of the residual gas in storage after completion of discharge, the more storage volume required at the storage facility or onboard the means of transport. This means that an even larger number of containers are required to provide a targeted working capacity or volume of cargo stored or delivered. One mitigating measure that limits the financial impact of heel in the cargo transport containers is that some of the heel gas may be used as fuel for a return trip or voyage. However, any residual cargo in excess of that which would be kept onboard ship for fuel is considered permanent heel that imposes a commercial penalty.
Therefore, a need exists for more efficient loading and discharge of compressed fluid cargos (e.g., CNG and PLNG), whether the cargo exists as a simple gas vapor, dense phase fluid, two-phase (vapor over liquid) fluid, or a liquid, such that mass and pressure of residual cargo gas volumes onboard the means of conveyance are minimized, allowing for any fuel supply that may be required for the trip returning to the source of the cargos. A need also exists for an efficient loading and discharge system that can function where no sufficient volume of displacement fluid is available at a cargo receiving facility.